An earth borer system of this type has been described earlier for instance in the publication EP-A-0 819 820. It includes drill rods which can be screwed together to extend the overall drill rod assembly. For drilling a bore hole, an additional drill rod is moved out of the magazine with the aid of the transport arm, attached to the drill rod assembly, screwed at one end to the rotary drive which for that purpose is retracted into a rearward home position, and screwed with its other end to the back end of the drill rod assembly. In analogous fashion, when upon completion of the drilling operation the drill rod assembly is withdrawn, the drill rods are unscrewed from the rear and the transport arm then moves them back into the magazine.
During the drilling process, the threaded junctions at the rotary drive and between the drill rods usually tend to be tightened beyond the original torque due to the considerable resistance of the subsoil to the mechanical rotation of the drill rod assembly. If both the effective drive torque and the resistance offered by the subsoil are greater than the static friction resistance between the contact surfaces of two drill rods, the threaded connection between these drill rods is tightened further. In the process, the drill rods are turned against the sliding friction resistance of their contact surfaces and are more strongly pressed together in an axial direction.
Given that the static friction resistance is greater than the sliding friction resistance, the torque required to unscrew the rods must be even higher than the torque with which the threaded rod connections are tightened during the drilling operation.
In order to have enough torque for unscrewing the drill rods with a resistive strength greater than the static friction, current systems employ a breakaway unit. In the system disclosed in EP-A-0 819820, a clamping unit is provided at the forward end of the frame, with a breakaway unit mounted immediately behind it.
If, for example, the drill rod assembly is to be extended by the addition of a drill rod, the first step is to stop the rotary drive in a forward retaining position on the frame. Then two hydraulically operated clamping jaws of the clamping unit are pressed against the back end of the rearward drill rod while similar clamping jaws on the breakaway unit grasp a drive chuck on the rotary drive. The clamping jaws of the breakaway unit are attached to a casing which by means of a hydraulic cylinder can be swiveled around the drill axis. For loosening the screw connection the hydraulic cylinder, turning the casing, applies a high torque on the drive chuck in a rotational direction opposite that selected for the drilling operation. This overcomes the static friction on the contact surfaces of the drive chuck and the rearmost drill rod and turns both elements by a certain angle in relation to each other.
The clamping jaws of the breakaway unit are then released and the rotary drive continues to turn the drive chuck with a relatively minor torque, thus completely unscrewing the threaded coupling. The rotary drive subsequently moves away from the forward direction into a rearward home position on the frame and, aided by two transport arms, inserts another drill rod.
The transport arms are multi-articulated gripping arms provided at their respective forward end with a hydraulically operated, more or less crescent-shaped grapple and a counter block. Actuating the hydraulic system allows the grapple to open up and, on grasping a drill rod, to close again. As the drill rod is moved between the rod magazine and the drill rod assembly, it is held between the inner surface of the grapple and the counter block.
A drawback of this prior-art design lies in the fact that the breakaway unit takes up much space so that an earth borer system of this type is unsuitable for drilling sites where there is not much room for accommodating the frame. Moreover, the design and operation of the transport arm in these earlier systems are rather complex and thus prone to breakdowns.